A compact desk-top laser-produced plasma source of soft X-rays for radiobiology research is presented. The source is based on a double-stream gas puff target and delivers nanosecond pulses of soft X-rays in the “water window” spectral range at a fluence of about 4.23´103 photon/µm2 per pulse on a sample placed inside the vacuum source chamber and about 2.60´102 photon/µm2 per pulse on a wet sample located outside the chamber in the He- environment. The source has been used to irradiate pBR322 plasmid DNA both in vacuum and the He-environment conditions. Single and double strand breaks were quantified by gel electrophoresis. The number of strand breaks increased with the increasing dose of the “water window” soft X-rays. The strand breaks of plasmid solution irradiated in helium condition may be associated with damage from water-free radicals.